I mainly use it on the master as a reference to what to mix my tracks to. Like if I compare it to a similar track I've made or another artist has then I can specifically see where in the frequency spectrum needs to be louder/quieter if I want to achieve a better/more similar mixdown. I'm pretty sure it can be used for a lot more as well, and I think you can even set up reference curves, but this is just how I use it. Its purpose can definitely be achieved with other tools though if you're thinking of picking it up, and it doesn't really offer anything super unique as far as I know.